feat: Integrate custom prop library and sandbox functionality

- Added support for custom props in the sandbox, including loading, saving, and rendering.
- Implemented `PropLibrary` for managing custom prop entries and scanning for `.prop` files.
- Introduced `PropVisuals` for shared visual and collision representation of custom props.
- Enhanced `StageSpawner` to handle spawning of custom props with layered visuals and collision hulls.
- Created `MissingPropPlaceholder` to manage missing custom props during scene reloads.
- Updated thumbnail generation to support custom props with unique keys based on file paths.
- Added tests for custom prop lifecycle, authored visual values, and missing prop handling.
- Refactored existing code to accommodate new prop functionalities and ensure backward compatibility.

scripts/missing_prop_placeholder.gd

@@ -0,0 +1,21 @@
+class_name MissingPropPlaceholder
+extends Node2D
+
+var original_entry: Dictionary = {}
+var missing_prop_path: String = ""
+const MARKER_SIZE := Vector2(180.0, 54.0)
+
+func configure(entry: Dictionary) -> void:
+	original_entry = entry.duplicate(true)
+	missing_prop_path = String(entry.get("asset_id", ""))
+	queue_redraw()
+
+func get_area_rect() -> Rect2:
+	return Rect2(-MARKER_SIZE * 0.5, MARKER_SIZE)
+
+func _draw() -> void:
+	var rect := get_area_rect()
+	draw_rect(rect, Color(0.76, 0.10, 0.10, 0.95), true)
+	draw_rect(rect, Color(1.0, 0.78, 0.78, 1.0), false, 2.0)
+	var asset_name := missing_prop_path.get_file()
+	draw_string(ThemeDB.fallback_font, rect.position + Vector2(10.0, 33.0), "Missing prop - %s" % asset_name, HORIZONTAL_ALIGNMENT_CENTER, rect.size.x - 20.0, 16, Color.WHITE)